I am going to pick up a 55g I got from craigslist and I am going to make my own cave for my kuhlis, bristlenose, and rams. I am going to be using rocks from the local lake (vinegar tested and boiled) and GE 1 100% Silicone. Will put up some pictures this evening.

In order to ensure your rock(s) are not limestone you pour a little vinegar on them. If they bubble, they are limestone and you cannot use them. If they don't bubble then you are good to go. The reason you can't use limestone is because it will mess with the pH of your water, make it more basic.

Here is my design. The only problem is the silicone did not want to stick well. I am going to try hot glue now.

In order to ensure your rock(s) are not limestone you pour a little vinegar on them. If they bubble, they are limestone and you cannot use them. If they don't bubble then you are good to go. The reason you can't use limestone is because it will mess with the pH of your water, make it more basic.

Here is my design. The only problem is the silicone did not want to stick well. I am going to try hot glue now.

Hot glue from a glue gun doesn't last very long under water, you could use some aquarium safe epoxy...

Hmmmm, I have some that has lasted almost a year. Although I can see why it wouldn't last too long. I may just use the hot glue to kinda keep everything in place. The rocks are heavy enough that I don't see my Bristlenose(my biggest fish) being able to move it.